It means lightning has been detected on or near the vicinity of the airport. When a Red Alert is issued it means they want all the ramp/ground workers to stay inside the terminals.
Interesting. I have never heard this type of alert on any US frequency. The closest weather related alert I have been hearing these thunderstorm-filled days is "Microburst alert, minus 35 knots detected one mile off the departure end of runway XX."
In fact, the other day I heard one humorous (to me, anyhow) sequence where inside of one minute three of these microburst alerts were given to an aircraft awaiting departure, and the pilot of the aircraft finally responded in a dead-pan voice, "Uh, we are just going to wait here until you stop giving alerts like those." It was the tone of the pilot's response that I had found humorous.
Of course, the actual recording is better than my recollection above.